Resistance to stress-corrosion cracking in nickel alloys

ABSTRACT

Nickel-chromium and nickel-chromium-iron-base alloys having an improved level of resistance to intergranular stress-corrosion cracking when in contact with aerated high-purity water at elevated temperatures. Alloys contain controlled amounts of aluminum, titanium, silicon and carbon. Grain size and alloy hardness correlated to further minimize stress-corrosion attack.

Uifie States gem (Copson et all. [4 1 Feb. 29, 1972 [54] RESISTANCE TOSTRESS-CGRROSIQN [56] References Cited CRACKING IN NICKEL ALLOYS UNITEDSTATES PATENTS [72] lnventors: Harry R. Copson, Mahwah, N.J.; SheldonDean, Jr Hamden Cmm 1,542,232 6/1925 Gmn ..7S/128 [73] Assignee: TheInternational Nickel Company, Inc., Primary Examiner-Richard 0. Dean NewYork, Attorney-Maurice L. Pinel 'l I [22] F1 ed May 26 1965 ABSTRACT[2]] Appl' 459050 Nickel-chromium and nickel-chromiumdrombase alloyshaving an improved level of resistance to inter-granular stress-cor-[52] US. Cl. ..75/l7l, 75/ 124, 148/1 1.5, rosion cracking when incontact with aerated high-purity 148/ 2, 1, 1 8/3 148/134 water atelevated temperatures. Alloys contain controlled [51] Int. Cl ..C22c19/00 amounts f l i titanium, ilicon and carbon. Grain size 0 Search134, and alloy hardness con-elated to further 5u-es5.co

sion attack.

6 Claims, 2 Drawing Figures RESISTANCE T STRESS-CORROSION CRACKING INNHCKEL ALLOYS The present invention relates to nickel-chromium andnickel-chromium-iron base alloys and, more particularly, to minimizingand/or overcoming the vexatious problem of stress-corrosion cracking ofsuch alloys when exposed to a high-purity water environment.

In recent years, what must be considered as a unique problem has beenreported in the literature as a result of laboratory tests (although thenumber of such reports has been relatively few) to the effect thatnickel-chromium and nickel-chromium-iron alloys, such as thosecontaining, inter alia, 75 to 80 percent nickel, 14 to 16 percentchromium and up to T percent or 8 percent iron, are prone to undergostresscorrosion attack in pressurized water of high purity attemperatures on the order of about 600 F., e.g., 570 to 660 F. Thatthere is a dearth of literature concerning the problem probably stemsfrom the fact that the conditions causing stress-corrosion cracking areunique and specific and are not normally encountered in service. Too, ithas been considered that for the most part nickel-chromiumiron alloyswere thought to be rather immune to this type of corrosive attack. Butthe possibility of such attack coupled with the gravity of the problemand the expansion of commercial operation dictates that efforts beindeed expended to thwart or minimize the same. Data presented hereinconfirm that the problem can arise and little emphasis need to addedregarding the importance and significance of this phenomenon.Deleterious cracking of components and vessels and the like used invarious systems of nuclear equipment wherein high-purity water is used,is indeed of no little moment.

Broadly speaking and as is well known to those skilled in the art,stress-corrosion cracking per se is a well known phenomenon. Over theyears, a wealth of literature coping with the problem ofstress-corrosion cracking of the austenitic nickelchromium stainlesssteels has been accumulated, particularly with regard to chlorideenvironments. While known recent avenues of approach for stainless steelwere considered in seeking a solution to the problem herein, it wasdeemed that little by way of substantive merit would be expected, sincewhat is apparently applicable to the stainless steels is not seeminglyapposite to the instant situation. Stress-corrosion cracking of thestainless steels in chloride solutions is primarily transgranular innature whereas the subject type of stresscorrosion cracking innickel-chromium-iron alloys is intergranular. This factor is indicativethat cracking is probably associated with some condition at the grainboundaries. Further, it has been found that AlSl 304 stainless steel hasupon expo sure to high-purity water of high temperature also sufferedstress-corrosion attack of the intergranular type in the sensitizedcondition.

Consideration was also given to the use of materials of very highpurity, i.e., the use of extremelypure nickel, chromium and iron and notmuch else. This pursuit appeals more to theoretical curiosity since itis basically impractical from a commercial viewpoint. Not only is itexpensive to use such materials but good commercial practice requiresthe use of various other elements to provide necessary deoxidizing andmalleabilizing attributes and to provide, for example, good forgingpractice. Moreover, it is not unlikely that the mechanical properties ofsuch alloys would be inferior.

The foregoing approaches serve to focus greater attention onappreciating the significance of the operating environmental conditionswhich could lead to the type of corrosive attack in question and alsounderstanding the ostensible peculiarities or behavior ofnickel-chromium-iron alloys upon exposure to such conditions. Perhaps itshould be mentioned that high-purity water as contemplated hereincontains not much above a total solids content of less than one part permillion (ppm) by weight and which has been distilled and/or deionized orotherwise treated such that it will manifest a specific resistance ofabout 500,000 ohm-cm. or higher. As is appreciated by those skilled inthe art, this type of water is used in atomic power equipment includingnuclear pressure vessels.

Certain environmental conditions have been established which eitherpromote or are causative of inducing or creating a propensity fordetrimental intergranular stress-corrosion cracking to occur innickel-chromium-iron alloys. Aerated high-purity water (in combinationwith the surface condition of the alloys) is one such condition andtemperature is another. Normally, high-purity water is devoid of oxygenand it is believed that the usual absence thereof has been responsible,to a considerable degree, for the lack of intergranular stress-corrosioncracking of nickel-chromium and nickelchromium-iron alloys heretofore ona commercial scale. But the possibilities of oxygen contamination areindeed more than sufficient to warrant the necessity of finding alloyswhich afford a markedly higher degree of resistance to such attack. Asto temperature, if the temperature of the water is at about roomtemperature, stress-corrosion attack does not appear to be much of aproblem. But, in commercial operation the tem-' perature of thehigh-purity water is normally above room temperature and is commonlyover 300 F., e.g., about 450 or 500 F., to about 660 F. and it is atsuch temperatures, particularly at the higher temperatures, where theoccurrence of intergranular stress-corrosion attack is most likely.

As to the condition of the alloys susceptible to attack and while littleis available covering the same in the published literature, it wouldappear that crevices" (in combination with aerated, high-temperature,high-purity water) exert a most pronounced subversive influence inproducing stresscorrosion cracking and other forms of corrosion. Whetherthe crevice by nature be a flaw, crack, sharp indentation or other suchsurface defect is rather inconsequential. The unfortunate fact remainsthat it is exceedingly difficult, if not impossible, to avoid or preventthe occurrence thereof. If the alloy is incapable of resistingstress-corrosion attack, there is also at least the likelihood ofgreater crevice buildup of corroded product. This would obviouslyinterfere in applications requiring moving parts and which of necessityrequire adherence to observing small clearance tolerances. Thus, it canbe reasonably stated without much reservation that the conditionsleading to the occurrence of intergranular stress-corrosion cracking ofnickel-chromium and nickel-chromium-iron alloys are at hand when aeratedhigh-purity water of a temperature of above about 300 F. is brought intocontact with certain alloys under stress and containing crevices and thelike. It is to be understood, however, that other factors areundoubtedly involved. The present invention is addressed to theforegoing specific problem.

It has now been discovered that certain nickel-chromium andnickel-chromium-iron alloys of special and controlled composition affordmarked resistance to intergranular stresscorrosion attack when suchalloys are brought into intimate contact with aerated high-purity waterat a temperature above about 300 F., e.g., 450, to about 660 F.,notwithstanding the fact that the surface of such alloys contains asurface defect, such as a crevice. it has been found that variouselements in certain amounts, notably aluminum, titanium and silicon,promote the susceptibility to such attack. in addition, it has also beenfound that the grain size and hardness of the alloys tend to influencethe behavior of the alloys in minimizing intergranular stress-corrosioncracking. An important attribute of the invention is that recourse tothe utilization of highly pure constituents in making the alloys isunnecessary.

it is an object of the present invention to provide nickelchromium andnickel-chromium-iron base alloys of special composition which manifest ahigh overall resistance to stresscorrosion cracking when such alloys arein contact with aerated high-purity water at a temperature above about300 F and up to at least about 660 F.

It is another object of the present invention to provide a process foraccomplishing the foregoing.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the accompanying drawing in whichthere is depicted a general relationship between hardness and grain sizeof alloys contemplated within the invention, curve A8 of FlG. ldepicting this relationship for alloys in the annealed condition andcurve CD of FIG. 2 illustrating this relationship for alloys in thesensitized condition.

Generally speaking and in accordance with the present invention,stress-corrosion cracking of nickel-chromium and nickel-chromium-ironalloys to be brought into contact with aerated high-purity water, thetemperature of the water being from above about 300 to about 660 F.,e.g., 450 F. or 500 to about 660 F., can be greatly minimized byutilizing alloys of the following most advantageous composition (basedon weight percent): about 14 to about 25 percent chromium, up to aboutpercent iron, e.g., l to 8 percent iron, aluminum in an amount up to0.05 percent, e.g., about 0.005 to 0.05 percent aluminum, titanium in anamount up to 0.1 percent, e.g., about 0.01 to 0.1 percent titanium,silicon in an amount up to 0.25 percent, e.g., about 0.01 to 0.25percent silicon, carbon in an amount up to 0.1 percent, e.g., 0.01 to0.1 percent carbon, and the balance essentially nickel. The use of theexpression balance or balance essentially in referring to the nickelcontent of the alloys, as will be understood by those skilled in theart, does not exclude the presence of other elements commonly present asincidental elements, e.g., deoxidin'ng and cleansing elements, andimpurities normally associated therewith in small amounts which do notadversely affect the novel characteristics of the alloys. in thisregard, it is preferred that elements such as phosphorus and sulfur bemaintained at low levels as is consistent with commercial prac tice.Manganese can be present in amounts up to at least 2 percent, e.g., 0.1to 1 percent.

In achieving optimum results and in addition to the aforementionedbalanced chemistry of the alloys, it is advantageous to control thegrain size and hardness of the alloys since relatively soft,fine-grained alloys tend to manifest greater resistance tostress-corrosion cracking. A desired correlation between grain size andhardness is generally reflected by the shaded area to the left of and/orbelow curves AB and CD of the attached drawing, i.e., the hardness andgrain size of the alloys are preferably interrelated such that theyrepresent a point lying within the aforesaid areas of the drawing.

Hardness and grain size, while dependent on the specific chemistry ofthe alloys, are also affected by heat treatment. High-temperatureannealing treatments with or without the application of cold rolling areconducive to coarse-grained structures, particularly where the alloysare thereafter subjected to a sensitizing treatment. For example, alloyscontemplated herein would often be welded to form a welded structure. Asa result of the welding operation, the alloys would pass through asensitizing temperature range of below about 1,500" to 800 F., e.g.,1,450t0 850 F. Thus, while the same alloy might manifest good resistanceto intergranular stresscorrosion cracking when in the annealedcondition, it might very well show cracking in the sensitized condition.Data presented herein establish that both conditions (annealed andsensitized) should be considered. Accordingly, it is thus beneficial inaccordance with the invention that while the annealing treatment can beconducted over a temperature range of about 1,500 to 2, 1 00 F, itshould be carried out at a temperature of from 1,500 to about 1,700 F.and most advantageously from 1,550 to l,650 F. At the low side of thetemperature range the alloys should be held at temperature for about 1to 2 hours or more whereas at the high side of the temperature range, e.g., 2,000 F., the alloys should be held at temperature for only a fewminutes. A most satisfactory treatment in achieving a fine grain size isto cold roll the alloys up to 50 percent reduction in thickness, e.g.,25 to 40 percent, and thereafter anneal at 1,550 to l,650 F. Thistreatment provides a grain on the order of about ASTM grain size No. 9or smaller and is further beneficial in that higher amounts of carboncan be employed, if desired, than otherwise might be the case. Coldrolling is advantageous since it results in attaining an elongated grainstructure and this type of grain is deemed more resistant tointergranular attack. The best condition is a soft, elongated andfine-grained, low-carbon alloy.

As has been indicated above herein, aluminum and/0r titanium and/orsilicon if used in excessive amounts lead to intergranularstress-corrosion cracking. lt has been found that even amounts of about0.07 percent aluminum, about 0.17 percent titanium and about 0.44percent of silicon, respectively, have led to cracking. However, it ismost important that these elements be present to provide gooddeoxidation, malleabilization and forging practices. Without thesefunctional characteristics, commercial processing is, as a practicalmatter, rendered more difficult and/or more costly. Tnis aspect lendsemphasis to the basic impracticability of using highly pure materials.It might also be mentioned that it is considered that high amounts ofaluminum and titanium promote high hardness and this in turn, asmentioned above, is conducive to stress-corrosion cracking.

Care should be exercised with regard to the carbon content of thealloys. Generally speaking, the limiting amounts of carbon present isinfluenced by grain size and heat treatment. It is deemed, however, thata basic criterion as to carbon content is in respect of the amount ofcarbon or carbide segregated at the grain boundaries in approximateinverse relation to the grain boundary area. Thus, where there is a fairnumber of carbides at a given grain boundary area of, say, X, crackingmight occur, whereas if the grain boundary area was 2X" or 3X, thesusceptibility to cracking would be greatly lessened. With properprocessing (cold rolling) and heat treatment to achieve a fine grainsize of about ASTM 7 or smaller, up to 0.15 percent carbon can beemployed. However, it is advantageous to maintain the carbon content ata level not greater than about 0.1 percent and preferably not greaterthan 0.03 percent to thereby minimize the occurrence of large amounts ofcarbon and/or carbide at a small grain boundary area.

Satisfactory results can also be achieved with alloys of the followingcomposition: about 14 to about 30 percent chromium, up to about 50percent iron, preferably not more than 25' percent iron, about 0.003 toabout 0.05 percent aluminum. about 0.005 to about 0.15 percent titanium,about 0.01 to about 0.3 percent silicon, about 0.01 to about 0.15percent carbon, and the balance essentially nickel, the nickelconstituting at least 30 percent and preferably at least 35 percent ofthe alloys.

For the purpose of giving those skilled in the art a betterunderstanding and/or appreciation of the invention, there is givenherein data illustrative of the advantages embodied by the alloys havingcompositions within the invention.

A substantial number of alloy test specimens were prepared havingcompositions given in Table 1 (Alloys A to M being outside the inventionand Alloys 1 to 7 being within the invention).

TABLE 1 Chemical composition Alloy No. Cr Fe C Si Ti Al Mn A 16.0 6.90.01 0.03 0.05 0.07 (1.01 B... 16.0 6.8 0.01 0.03 0.06 0.56 0.01 C...15.6 6.7 0.01 0.03 0.09 2.9 0.01 -D.. 15.3 7.5 0.03 0.03 0.17 0.02 0.0116.1 6.4 0.05 0.19 0.26 0.07 0.15 16.0 7.2 0.05 0.10 0.32 0.02 0.29 G..15.7 7.1 0.02 0.10 0.35 0.01 2.0 11.. 16.3 6.8 0.10 0.03 0.96 0.02 $0.01I... 15.1 8.2 0.06 0.15 I 2.4 0.09 2.1 .l 16.0 6.8 0.03 0.15 3.0 0.020.30 K... 15.7 7.0 0.01 0.02 3.6 0.03 0.06 L... 16.0 6.8 0.01 0.44 0.010.02 0.03 M.. 15.9 7.2 0.05 0.03 0.01 0.01 0.01 *1 15.9 6.7 0.02 0.040.05 0.03 0.04 I 15.9 6.7 0.02 0.04 0.05 0.03 0.04 3 16.0 6.8 0.01 0.250.01 0.02 0.03 4 16.3 7.2 0.01 0.02 0.08 0.02 0.07 5 16.0 6.8 0.01 0.020.01 0.01 0.26 6 16.0 6.8 0.01 0.02 0.01 0.01 0.54 7... 15.7 6.6 0.010.02 0.01 0.01 2.2

These alloys were prepared using vacuum melting techniques and usingmaterials of relatively high purity. The alloys were cast as -poundingots. After removing surface defects, the alloys were heated to 2,200F. and forged to flats (1 inch by 3.5 inches by 10 inches). Afterreheating to 2,150 F., the flats were hot rolled to a thickness of about0.2 inch. Subsequent to conventional processing, including cold rollingto provide specimens about 0.12 inch thick, the alloys were subjected toheat treatment. Two different heat treatments were employed, oneconsisting of solution treating at about 1,950 F. for onequarter hourfollowed by a water quench. A high-solution treatment temperature wasdeliberately employed to add to the severity of the test. The secondtreatment consisted of a sensitizing treatment whereby the specimen washeated to a temperature of about 1,300 F., held at this temperature forabout one hour and then air cooled. Thus, two specimens (strips) of eachalloy composition were prepared, one being subjected to the solutionanneal treatment, the other being subjected to the sensitizingtreatment. it was deemed necessary to test the alloys using thesensitizing heat treatment since, as referred to above herein, thealloys would be often used in this condition.

Standard autoclave testing techniques were employed and duplicate U-bendtest specimens of each alloy were used.

These specimens were formed by bending two flat strips simultaneouslyaround a mandrel and inserting a bolt through the parallel legs thereof.The flat strips were about three-eighths inch in width, 0.l2-inch thickand about 3.25 inches long. Prior to bending about the mandrel, an areaabout 0.005 inch deep and about 1 inch long was ground in the centerportion of one strip. On bending about the mandrel, a tapered crevicewas formed.

The test solution was aerated high-purity water (air saturated at oneatmosphere) with the pH thereof having been adjusted to about pH 10.0.This test solution was placed in the autoclave and a head space havingadditional air was maintained. The test specimens were immersed in thesolution and the autoclave sealed and brought to a test temperature ofabout 600 to 660 F. The autoclaves were opened about every 2 weeks andthe specimens inspected for cracks, whereafter the tests were restartedwith fresh solution in those instances where cracking was not visuallyobserved. The tests were conducted generally over a period of 8 weeksand both visual and metallographic examinations of the specimens weremade. The results are reported in Table 11 wherein the numerals indicatethe time of cracking; for example, the numerals 2 and 4 indicate thespecimens cracked within the first and second 2-week periods,respectively. It should be mentioned that if a specimen did not exhibitvisual cracking, the metallographic examination was not then made;however, any specimen which did not manifest visual cracking during thefull 8-week period was thereafter examined metallographically.

As illustrated by the data, a fine grain size is insufiicient. Thus,Alloys H, I and K all had a grain size of ASTM No. 8 or finer andcracking occurred. In each of these three alloys, the titanium contentwas too high and cracking occurred regardless of the hardness, grainsize relationship. Low hardness values did not necessarily result inaffording resistance to intergranular stress-corrosion cracking. This isreflected, for example, by Alloys B, C and F which contained excessiveamounts of aluminum (Alloys B and C) and titanium (Alloy F), wherein theannealed hardness level did not exceed 148 VHN. Alloys A, D and Ladditionally illustrate that aluminum, titanium and silicon in amountsas low as 0.07 percent, 0.17 percent and 0.44 percent, respectively,notwithstanding that the relationship between hardness and grain sizemight be represented by a point within shaded areas of curves AB and/orCD of the drawing, are conducive to cracking. Alloys E, G and J followeda rather similar pattern. Alloy M indicates that when the hardness,grain size relationship defined by curves AB and/or CD is not satisfied,cracking can ensue although the chemistry of the alloy might be withinthe compositional ranges described herein. However, Alloy M' respondedrelatively well and with a lower carbon content (Alloys 1 to 7) it isconsidered that greater resistance to stresscorrosion cracking would beconferred. A review of all the data concerning Alloys A through Mreflects that when the alloys are in the sensitized condition, there isa greater susceptibility to premature cracking. This is, of course, alsoindicated by the curves AB and CD.

in contrast to Alloys A through M, Alloys Nos. 1 through 7 performedsatisfactorily under the same test conditions. In each case, thecomposition of Alloys Nos. 1 through 7 is within the scope contemplatedherein as is also the preferred relationship between hardness and grainsize.

It is to be observed that the present invention provides nickel-chromiumand nickel-chromium-iron alloys highly resistant to intergranularstress-corrosion cracking when in contact with pressurized, aeratedwater at a temperature of above about 300 to about 660 F.,notwithstanding that the surface of the alloys be characterized by acrevice or some such similar surface defect. The invention is alsoapplicable in minimizing intergranular stress-corrosion cracking inaerated high-purity water at surface areas which do not contain obvio uscrevices yressure vessels, heat exchangers, steam genera- Table AnnealedSensitized Cracking time Cracking time Gram Metallo- Grain Metallo-VHN** size Visual graphic VHN size Visual graphic Am. 143 4 OK. OK OK,OK 137 3.5 OK, OK C*, OK 13. 141 6.5 8. OK C, OK 145 6 OK, OK OK, OK C.148 5 4. OK C, OK 200 4 OK C D. 142 5.5 OK, OK C", OK 147 5 OK, OK OK,OK E. 151 5.5 OK, OK OK, OK 148 4 4 C, C F. 139 4.5 8, OK C, OK 141 4.54, OK C, OK G. 154 4 OK, OK OK, OK 148 5.5 4, OK C, OK H. 223 9.5 OK, OKOK, OK 207 9.5 8, OK C, OK 1. 225 9 OK, OK C, OK 260 9 4, OK C .1 176 7OK, OK C, OK 271 6.5 4, OK C, OK K. 236 9 OK, OK OK, OK 313 8 6, OK C,OK L. 149 5 OK, OK C, OK 140 5 OK, OK OK, OK M 155 5 OK, OK OK, OK 154 68, OK C, OK 1. 156 6.5 OK, OK OK, OK 138 5.5 OK, OK OK, OK 2 136 5 OK,OK OK, OK 136 5 OK, OK OK. OK

132 3.5 OK, OK OK, OK 132 2.5 OK, OK OK, OK

144 5 OK, OK OK, OK 141 5 K, OK OK, OK

135 5 OK, OK OK, OK 130 5 OK, OK OK, OK

158 6 OK, OK OK, OK 143 6.5 OK, OK OK, OK

139 4 OK, OK OK, OK 133 5 OK, OK OK, OK

** Average hardness value.

OK No attack in eight week test.

C Cracks extended more than L, through specimen. C*= Shallow cracksusually about 1 to 2 grains deep.

tion surfaces, tubing, etc., are illustrative of the of articles whichcan be fabricated from the alloys of the invention. However, the presentinvention should not be confused with nickel-chromium andnickel-chromium-iron alloys of the agehardening type and which containsubstantial amounts of precipitation hardening ingredients such asaluminum and titanium. The alloys of the present invention are, as apractical matter, of the nonage hardening type.

As used herein and as is, of course, well known to those skilled in theart, the term annealed condition means the condition of the alloy uponcooling from the solution annealed condition (often referred to assimply the annealed condition). Similarly, sensitized condition refersto the condition of the alloy after cooling from a sensitizingtreatment. it should also be mentioned that the shaded areas defined bycurves AB and CD can also be expressed as the area to the left of and/orbelow the curves AB and CD.

Although the present invention has been described in com I junction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. A nickel-chromium base alloy characterized by an improved level ofresistance to intergranular stress-corrosion cracking when subjected tocontact with aerated high-purity water at a temperature of about 300 toabout 660 F said alloy consisting of about 14 to percent chromium, up to10 percent iron, aluminum present in an amount up to 0.05 percent,titanium present in an amount up to 0.1 percent, silicon present in anamount up to 0.3 percent, carbon in an amount up to 0.1 percent and thebalance essentially nickel.

2. An alloy as set forth in claim l in which the aluminum is present inan amount of 0.005 to 0.05 percent, the titanium ispresent in an amountof 0.01 to 0.1 percent, the silicon is present in an amountof 0.01 to0.25 percent and the carbon is I condition it is represented by a pointfalling within the shaded area defined by curve CD.

3. A process for providing a nickel-chromium base alloy characterized bygood resistance to intergranular stress-corrosion attack when in contactwith aerated high-purity water at a temperature of about 300 to about660 F. which comprises establishing a molten bath containing chromium,iron, aluminum, titanium, silicon, carbon and the balance essentiallynickel, controlling the amounts of the respective constituents withinthe following ranges: about 14 to 25 percent chromium, up to 10 percentiron, aluminum present in an amount up to 0.05 percent, titanium presentin an amount up to 0.1 percent, silicon present in an amount up to 0.25percent, carbon in an amount up to 0.03 percent, and the balanceessentially nickel, v

until the molten bath has solidified; forging the alloy and thereaftercold rolling the alloy to effect a reduction of up to about 50 percentin thickness and thereafter solution annealing said alloy at atemperature of about 1,500to l,700 F r such a metal article, the articlebeing formed from an alloy in accordance with claim 1.

6. A process as set forth in claim 5 in which the article is formed froman alloy in accordance with claim 2.

2. An alloy as set forth in claim 1 in which the aluminum is present inan amount of 0.005 to 0.05 percent, the titanium is present in an amountof 0.01 to 0.1 percent, the silicon is present in an amount of 0.01 to0.25 percent and the carbon is present in an amount of 0.01 to 0.03percent, the hardness and grain size of the alloy being correlated suchthat when the alloy is in the annealed condition it is represented by apoint falling within the shaded area defined by the curve AB of theaccompanying drawing and when the alloy is in the sensitized conditionit is represented by a point falling within the shaded area defined bycurve CD.
 3. A process for providing a nickel-chromium base alloycharacterized by good resistance to intergranular stress-corrosionattack when in contact with aerated high-purity water at a temperatureof about 300* to about 660* F. which comprises establishing a moltenbath containing chromium, iron, aluminum, titanium, silicon, carbon andthe balance essentially nickel, controlling the amounts of therespective constituents within the following ranges: about 14 to 25percent chromium, up to 10 percent iron, aluminum present in an amountup to 0.05 percent, titanium present in an amount up to 0.1 percent,silicon present in an amount up to 0.25 percent, carbon in an amount upto 0.03 percent, and the balance essentially nickel, until the moltenbath has solidified; forging the alloy and thereafter cold rolling thealloy to effect a reduction of up to about 50 percent in thickness andthereafter solution annealing said alloy at a temperature of about1,500*to 1,700* F.
 4. The process set forth in claim 3 wherein thecomposition, cold rolling and solution treatment are controlled suchthat the relationship between the hardness and grain size of the alloyis represented by a point falling within the shaded area defined by thecurve AB of the accompanying drawing.
 5. A process for minimizingintergranular stress-corrosion cracking of metal articles in contactwith aerated high-purity water at a temperature of about 300* to about660* F. which comprises flowing high-purity water past and in contactwith such a metal article, the article being formed from an alloy inaccordance with Claim
 1. 6. A process as set forth in claim 5 in whichthe article is formed from an alloy in accordance with claim 2.